Interpreting Earth’s Rocks to Determine Its Past History

Geologic Timechapter 18

• Relative Dating - sequence of geologic events in a region (order of past events)

• Not quantitative

http://www.angelfire.com/az3/mohgameil/Layering4.jpg

Law of Superposition

• Oldest layer on bottom (if undisturbed)• Only works for SED rocks

Younger

Older

Law of Superposition

In horizontal rock layers, the oldest and first formed is on the bottom. 5 is the youngest

because it is on the top, closest to the surface.

http://www.calstatela.edu/faculty/acolvil/sediment/superposition.jpg

The Grand Canyon

Principle of Original Horizontality

• Sediments are originally deposited as horizontal (or near horizontal) beds (ONLY FOR SED ROCKS)

Folding happened AFTER rocks were deposited

folded rock layers = compression by tectonic activity

Cross-Cutting Relationships(overlap law)

• An igneous intrusion (or fault) is younger than the rock layers that it cuts across.

fault

oldest

These are younger than

the surrounding

rock

youngest

Cross-Cutting Relationships

6 is younger than1, 2, or 3 because it cuts across them!

1

12

2

3

3

4

5

Which came first?

A

CB

DE

Order the rock layers above from oldest to youngest.

Which came first?

The first layer deposited and the oldest is D, followed by C, A, B, and E.

DD

DC

CA

B

B

E

Law of inclusions

• The inclusion (xenolith) is older than the rock it is in

• Snickers bar

James Hutton: The father geology (1726-1797)

Uniformitarianism: processes acting on earth today (volcanism, erosion, plate tectonics, glaciation, etc.) have done so in the past IN THE SAME WAY!

“The present is the key to the past.”

Unconformities = “Missing Time”

• Represented by squiggly lines on cross sections

Angular unconformity

Angular Unconformities

• Folded layers below horizontal layers.

angular unconformity

(rock layers have been eroded away)

rock layers are eroded

new horizontal layers are deposited

F E D C B A

Angular Unconformity

sedimentary rocks above and below unconformity surface with rocks below tilted at an angle

Granite

sedimentary rock layers

Cambrian

Precambrian

Nonconformity – younger sedimentary rocks rest on older igneous rocks

Nonconformity

Disconformity

DISCONFORMITY:Missing rock layers (sedimentary)

Very hard to pick out

Disconformities

Let’s Play….Name the UNCONFORMITY!

Siccar Point, Scotland

http://www001.upp.so-net.ne.jp/fl-fg/06-02.htm

Resources• http://courses.geo.ucalgary.ca/glgy209L03/docs/Lecture15_GLGY209_six.pdf• http://perth.uwlax.edu/faculty/stoelting/Intro/Guides/Images2/law_of_superposition_b.jpg• http://www.science.sjsu.edu/scied/255/ppt/superposition/img025.jpg• http://www.gpc.peachnet.edu/~jsummero/Cross-cutting-a.jpg

• http://www.gc.maricopa.edu/appliedscience/gjc-nsf/images/CRH0204.JPG• http://maverick.gc.maricopa.edu/appliedscience/gjcweb/geomap/images/ang.unconf.jpg• http://www.geog.ouc.bc.ca/physgeog/contents/images/hutton.jpg• http://www.bigbendpaleo.com/Photo%20Gallery/Big%20Bend%20Region,%20Texas/Big%

20Bend%20National%20Park%20-%20Syncline%20-%20Javalina%20Creek%20-%2020%20June%201995.jpg

• http://geoimages.berkeley.edu/GeoImages/Johnson/Images/Large/CD3/028.jpg• http://www.vvdailypress.com/travel/rainbow/images/rock.jpg• http://academic.emporia.edu/aberjame/struc_geo/folds/fold06.jpg• http://www.geology.pitt.edu/images/folds.jpg• http://faculty.evansville.edu/de3/b39902/PPoint/Geologic%20Time/img007.jpg• http://www.photo.net/photo/pcd2882/grand-canyon-region-aerial-85.3.jpg• http://www.cowanstark.com/horsehoe-bend.jpg• http://home.flash.net/~alanm52/Cross_cutting.jpg• http://www-class.unl.edu/geol101i/images/igneous%20images/Black%20Canyon%20Gunn

ison.jpg• http://freespace.virgin.net/craven.pendle/stratigraphy/unconformity.jpg

• http://comp.uark.edu/~sboss/siccar.jpg• http://www.pittstate.edu/services/scied/Teachers/Field/Camp/Us67-1/us67cut.jpg• http://maverick.gc.maricopa.edu/appliedscience/gjcweb/geomap/images/MW8-11.jpg• http://www3.uakron.edu/geology/Foos/parks/rmgcan2.html• http://www.ldeo.columbia.edu/edu/dees/ees/ies2/slides/disconformity.gif• http://geology.lssu.edu/NS102/images/figure%2009-06.gif• http://www.crienterprises.com/C-Assets/J-Hanifa-Unconf-W-Birk.jpg• http://www.uoguelph.ca/~sadura/stgref/stg32b.gif• http://www.geo-outdoors.info/images/eye_protection.jpg• http://enterprise.cc.uakron.edu/geology/natscigeo/Lectures/time/gtime1.htm